Just to clarify my suggestion:
USSR may not initiate a war with Japan if it is already at war with Germany or Italy
USSR may not initiate a war with Germany or Italy if it is already at war with Japan
Japan may not initiate a war with USSR if it is already at war with UK/FRANCE/USA
Japan may not initiate a war with UK/FRANCE/USA if it is already at war with USSR
The Japan/China situation does not effect these.
In all cases the bind ends when a capital (or both capitals) of the enemy you are already at war with fall(s).
It will be seen from this that if the game follows the historical pattern, then USSR & Japan will be trapped in a double-bind preventing them from becoming at war with each other until either the Euro Axis or the Western Allies are defeated.
I’m thinking about similar restrictions on other powers, for example should UK or France be allowed to declare war on Japan while any of London/Paris/Rome/Berlin is Axis controlled?
Should Germany and Italy be automatically drawn into war with the USA if Japan attacks it?
Would Italy declare war on ANY other power unless Germany had already done so?